Backport PR #54789 on branch 2.1.x (WARN: Remove non-actionable warning in value_counts) (#54795)

Backport PR #54789: WARN: Remove non-actionable warning in value_counts

Co-authored-by: Patrick Hoefler <61934744+phofl@users.noreply.github.com>

Author: meeseeksmachine

pandas/core/frame.py

@@ -7260,7 +7260,7 @@ def value_counts(
             subset = self.columns.tolist()
 
         name = "proportion" if normalize else "count"
-        counts = self.groupby(subset, dropna=dropna).grouper.size()
+        counts = self.groupby(subset, dropna=dropna, observed=False).grouper.size()
         counts.name = name
 
         if sort:

pandas/tests/frame/methods/test_value_counts.py

@@ -175,3 +175,17 @@ def test_data_frame_value_counts_subset(nulls_fixture, columns):
     )
 
     tm.assert_series_equal(result, expected)
+
+
+def test_value_counts_categorical_future_warning():
+    # GH#54775
+    df = pd.DataFrame({"a": [1, 2, 3]}, dtype="category")
+    result = df.value_counts()
+    expected = pd.Series(
+        1,
+        index=pd.MultiIndex.from_arrays(
+            [pd.Index([1, 2, 3], name="a", dtype="category")]
+        ),
+        name="count",
+    )
+    tm.assert_series_equal(result, expected)